Answering the OP's questions, from my own head (and therefore probably no more reliable than AI.....)
1. Yes, an M is a stronger machine than an L, they're two sizes up in frame.
2. They also don't come with an HST, which is a pro or a con depending on what you're doing. Unless you need the additional capacity of an M, I personally would prefer an HST, given you mention a lot of loader work. But that's a religious debate, and plenty on here don't like HSTs
3. I don't think the M is less complex than an L. The L is an economy machine (if you're looking at the standard L, rather than the Grand L), and mechanically reasonably simple. The M is a bigger machine with more stuff going on. Still not complex as such, but definitely more features.
4. The MX is the machine in between - the sizing goes LX, then L, then Grand L (a -40 or -60 L series), then MX, then M. The MX is as big as you can go with an HST. I think MX is a more recent addition to the lineup, first made in 201x I think. Whereas the L and M have been made for 20-30 years in one form or another.
In short, for me, the M is a bigger machine with more features. Often a shuttle shift, usually will have hydraulic outlets, often a more complex 3ph with features like draft control. Usually cat 2 3ph I think. None of that is bad, it's just a lot more machine than an L. I'd recommend working out what size machine you need (or want), then buy that. I wouldn't choose between an L and an M based on perceived complexity.